AWS provides AWS Organizations to centrally manage and govern your environment.

Plan & Execute

• Go to AWS Console-> AWS Organization.
• Define an AWS Organization to centrally govern your environment
• Create a root organization and sub organizations within the root organization to manage projects.

Best practices

• Create multiple sub organizations (i.e. development, testing, production) and client specific
  sub organizations.
• Don’t allocate any resources to the root account
• Get visibility of billing per sub organizations and setup alerts for budget controls.
•  Use Principle of Least Privilege with IAM (i.e. allocate minimal access to perform tasks) to setup users for respective accounts.

With the above setup, you can easily monitor your cloud spends per project and optimize it accordingly. You could also centrally manage and audit trail your accounts using AWS CloudTrail.